1. Field of the Invention
The invention relates to the treatment by a biobarrier- or biofilter-type process of an effluent polluted by MTBE or TAME, using bacteria capable of degrading methyl tert-butyl ether or tert-amyl methyl ether.
It relates more particularly to the use in such a process of a new type of support which serves to fix the bacteria. It applies particularly to the industry for the treatment of water and of water-bearing soils and grounds.
2. Description of the Prior Art
It is known that methyl tert-butyl ether (hereafter called MTBE) or tert-amyl methyl ether (hereafter called TAME) are ethers that can be used as oxygenated additives in unleaded gasolines with the aim of increasing their octane rating. The increasing use of additives such as MTBE or TAME results in large volumes being stored and transported, in particular mixed in gasolines. Due to the relatively high frequency of leaks recorded in underground gasoline stores (service stations in particular), the soluble compounds of the gasolines then dissolve in the aquifers on contact. This is the case in particular with MTBE, which is very soluble in water (40 g.L−1) but also TAME. The presence, in particular of MTBE, in underground water collection wells manifests itself for the consumer in a disagreeable smell that is perceptible even at low concentration. Moreover, the mobility of these compounds means that the pollution “trails” are very broad. These two points, together with the public-health problem posed by the possible effects of the ingestion by the consumer of water polluted by MTBE or TAME at low concentration over extended periods means that the implementation of the clean-up processes is envisaged.
It is in this particular case because of the physico-chemical properties of these compounds that physico-chemical clean-up processes prove pretty ineffective and processes using aerobic microorganisms with particular capacities for degrading MTBE or TAME are envisaged. Such microorganisms are rare and their rates of growth on MTBE, in particular, are slow. It is therefore particularly useful to define a way of using these microorganisms which optimizes their particular capacities. Recent literature shows that the biobarrier is where the use of the pollutant as a source of carbon and energy through the microflora used constitutes a clear advantage, and this technique is wholly suitable for effectively introducing the selected microflora into the aquifer to be decontaminated. In this case, a trench is dug at the base of the aquifer polluted by the MTBE or TAME, then a support, in general of mineral type, is put in place and the aerobic microorganisms of interest (degraders of MTBE and/or TAME) are fixed on said support to clean up the aquifer during its natural or forced flow through the biobarrier. More precisely, the bacterium can be developed on a biofilter or biobarrier system of suitable volume, the effluents containing said ethers are introduced in the presence of air or oxygen into the biofilter or into the biobarrier at an MTBE flow rate of less than 30 mg.L−1.h−1 and the effluent contains a small concentration of said ethers.
For example, Steffan et al. [2001] successfully used the selected ENV425 strain by bioaugmentation in contaminated aquifers in conditions (injection of air to which propane had been added) allowing the strain to degrade the MTBE in situ by cometabolism. Similarly, Salanitro [2000] used an improved culture of the BC-1 consortium to carry out a pilot-scale operation for rehabilitation of a sheet of water polluted by MTBE. The treatment used a biobarrier 6 m long supplied with oxygen. The support was composed of the sand of the formation, which was of an average granulometry. After 261 days, the concentration of MTBE had fallen from 2-9 mg L−1 to a value below 10 μg L−1 in the zone that had benefited from bioaugmentation. The concentration of TBA was also below 10 μg L−1. An effective, but clearly slower, decrease in the concentration of MTBE was observed in the control zone stimulated only by added oxygen.
There is described in French patent application FR-A-2 830 854 the isolation of a bacterium, Mycobacterium sp. I-2562, hereafter identified as Mycobacterium austroafricanum I-2562 or M. austroafricanum I-2562, capable of thriving on MTBE or TAME as sole source of carbon and energy leading to a complete degradation of the MTBE and its degradation intermediates, tert-butyl formate or TBF and tert-butyl alcohol or TBA, into carbon dioxide and water and also to the production of biomass or even to a complete degradation of the TAME and its degradation intermediates, tert-amyl formate or TAF and tert-amyl alcohol or TAA, into carbon dioxide and water and also to the production of biomass. The addition of yeast extract during the growth on MTBE, in particular, had a positive effect on growth, improving the rate of degradation of the MTBE although the yeast extract alone did not constitute a growth substrate for the strain.